megane 2005 aircon?

hi i have a megane 1.5 dci air con dont work had it regased told it was regased but still dont work
ive done some investigating and found when i press aircon button no aircon. but if i take off the plug to the pessure switch. then put it back on while aircon is on. it starts the fans then after 5 secs the pump starts and all works well. nice and cold and if i switch off with button in car and on again starts working again. but if turn egnition off and start again it dont work again untill i pull the plug again. tryed a pressure switch from scrap yard still the same. any sugestions anyone? ie how to test pressure switch ect